A turbocharger rotates an exhaust turbine by utilizing exhaust gas that flows through an exhaust channel and drives a turbo compressor coupled to the exhaust turbine, thereby supplying air to an internal combustion engine (supercharging). In relation to such a type of turbocharger, a two-stage turbo system is known, where a turbocharger is provided for each of the high-pressure side and the low-pressure side to perform two-stage supercharging to improve supercharging efficiency.
For instance, Patent Document 1 discloses a two-stage turbo system including a high-pressure stage turbocharger and a low-pressure stage turbocharger, wherein a high-pressure stage exhaust bypass valve, disposed in a high-pressure stage bypass flow passage that bypasses the high-pressure stage turbocharger, is configured to open in response to an increase in the flow rate of exhaust gas, whereby the operational range transitions from an operational range of the high-pressure stage turbocharger that uses the high-pressure stage exhaust bypass flow passage to an operational range of the low-pressure stage turbocharger that does not use the high-pressure stage exhaust bypass flow passage.